In the art of fabricating tube and shell heat exchangers, it is well known that a proper seal and support structure is required at each header plate-core tube interface. Even minor leaks at the tube joints will impair the function of the heat exchanger. Prior inventions have taken two approaches to establishing such a connection at this interface. The first approach was via a purely mechanical connection. The second approach has been via a metallurgical bonding, particularly brazing.
The first noted approach to be discussed is the method of mechanically sealing this interface. Prior art, such as U.S. Pat. No. 4,152,818 to Mort et al., sets forth an example of such a technique. The mechanical sealing process first involves inserting an end of the core tube into a hole in the header plate. A rivet is then inserted into the core tube end and expansion of the rivet subsequently creates a high load friction connection end of the core tube into a hole in the header plate. A rivet is then inserted into the core tube end and expansion of the rivet subsequently creates a high load friction connection between the core tube and the header plate. This resulting connection serves as the joint for the interface. Several methods can be used to expand the rivet, but a complete expansion requires all contact areas between the core tube and header plate to be a maximum of 0.001 inch to intimate. In order to provide this complete seal over 100% of the interface, several process steps may be required. Even with a 100% complete seal, varying load forces can damage this mechanical seal. For example, vibrations and pressure fluctuations may cause one of the header plate and core tube to move relative to the other. In order to assure completion of the seal, an added step, as shown in prior art U.S. Pat. No. 4,482,415 to Mort et al., of using a sealant material at each header plate-core tube interface can be used. In this type of process, the joint is codependent on the mechanical process and the sealant process.
Another approach for sealing this joint is via a braze joint construction. Prior art, such as U.S. Pat. No. 4,207,662 to Takenaka, sets forth an example of using clad braze materials for this process. In such a process, the clad braze material is located on the exterior surface of one of the objects to be joined. For example, a core tube is inserted into a hole in a header plate having braze material located on at least one of its sides. Upon brazing, the clad material melts and forms the joint. Alternatively, the core tube could be clad with braze material. However, clad materials are typically produced as flat stock and the products shaped therefrom, for example the header plate or core tube, is preferably also flat in order to satisfactorily retain the clad material. During brazing, the clad braze material melts and, like any liquid, will flow and take the path of least resistance. With a flat surface, it is therefore difficult to direct the flow of the melted braze material. In order to overcome the difficulties in directing the flow of melted clad material, the previously mentioned prior art patent sets forth an example of using a flat, inclined surface to direct the flow of the melted material towards the intended area of joining.
Another prior art braze joint construction approach involves the use of diffusion bonded braze material. Typically such a manufacturing process first includes the initial diffusion bonding of a braze foil alloy to the header plate in order to bond the braze material in place. The core tubes are then inserted into holes in the header plate, followed by a mechanical staking operation of the tube ends in order to form a clearance controlled or intimate bond at the header plate-core tube surface. Subsequent vacuum brazing is then employed to bond the tube end to the header plate. An intimate bond is critical to any brazing operation. It is the intimate contact between the header plate and core tube that promotes the wetting of the joint surfaces with the braze alloy. A mechanically staked core tube, though, exhibits overall distortion due to the biaxial (radial and axial) stressing of the tubes that occurs during the noted staking operation. The core tube staking process is generally performed manually, and in addition to it being labor intensive, is largely uncontrolled thus introducing excessive process variations and large compressive stresses in the core tubes. This process often creates product rejections ranging from braze joint leaks to unacceptable dimensional distortions. In addition, the braze alloy diffusion bonding process is dependent on a complex vacuum process and often produces unacceptably low yields. The diffusion bonding process also produces changes in the aluminum header plate material, via diffusing out the silicon, which has a negative effect on the brazing process as well as the header plate material.
Various other methods have been used in order to create the critical intimate contact between the bonding surfaces. Prior art, such as U.S. Pat. No. 3,496,629 to Martucci et al., teaches welding the core tube to the header plate in order to produce the intimate contact area.
Another example of a prior art brazing technique is set forth in U.S. Pat. No. 5,464,145 to Park et al. This technique does not address the need for an intimate contact area between the bonding surfaces. Other prior art brazing techniques are in U.S. Pat. No. 2,267,315 to Stikeleather and U.S. Pat. No. 5,507,338 to Schornhorst et al. These two references set forth a process of joining the tubes to each other, but not to the header plate. A further reference, U.S. Pat. No. 6,170,738 to Otsuka et al. sets forth the use of a specific material for brazing low-melting point aluminum material parts.